198 CHARLES HARLAN ABBOTT 



however, they accidentally start to wander out from dark cavi- 

 ties, a definite phototactic reaction probably turns them back 

 into the dark again. This behavior is difficult to observe in the 

 field, and analysis is, moreover, complicated by reaction to 

 contact. 



Simple experiments show that land isopods have a phototactic 

 response to light, even when the light is somewhat diffuse. For 

 example, at 4 P.M. on January 19th, when the light entering the 

 room through a north window was becoming rather dim, indi- 

 viduals of Oniscus, placed on a table several feet from the win- 

 dow, definitely and consistently turned away from the light 

 and traveled in the opposite direction. Such experiments show 

 that these animals have a definite negative phototaxis in ordinary 

 daylight. 



The experiments in the following sections are devoted to 

 analysis of this reaction. 



3, Vision 



Land isopods have rather primitive compound eyes, and are 

 to be classed among the animals which have the beginning of 

 vision. As they live constantly in the dark, it does not seem 

 probable that vision plays any appreciable part in their normal 

 life. Observation of the use of the antennae in ordinary iso- 

 pod activity indicates that vision is of much less importance 

 than reaction to contact in the ordinary life of these animals. 



B. RELATION OF CONTACT AND VISION 



The antennae, the most important contact organs of the 

 isopod, are in constant use during ordinary locomotion. They 

 are extended in front of the head and the tips are repeatedly 

 touched to the substratum. The animal often pauses and waves 

 its antennae about in the air or rubs them over any object which 

 chances to be in its path. The antennae are used to test the 

 nature of the environment in a way similar to that in which a 

 blind person, by passing the finger-tips over objects and by the 

 use of a cane when walking, uses the sense of contact as a sub- 

 stitute for the sense of sight. 



